Green coal for power - To take care of post-Kyoto issues from energy security point-of-view:
a. Coal is the world’s most abundant and important source of primary energy. Turning a potential pollutant into a clean, green fuel for economical power production has become a matter for concern on a global scale. Coal continues to dominate the energy industries as the single most important and widely-used fuel. Delivering around 27 per cent of the world’s consumption of primary energy, almost half of which is used for electricity generation; reserves of coal are spread worldwide throughout some 100 developed and developing countries, sufficient to meet global needs for the next 250 years.
b. Although a combination of economic and environmental pressures has forced the closure of older, inefficient, fossil fuelled thermal stations, the massive growth in power demand on a world scale will continue to be met predominantly by coal-fired plant for the foreseeable future. In many of the rapidly developing and industrializing regions of the world the rate of consumption of coal as a primary fuel for electricity generation is actually increasing. In energy-hungry
c. In contrast with both natural gas and LPG, hard coal can contain a wide range of compounds including sulfur in addition to useful hydrocarbons. The percentage of sulfur can vary widely, with relatively low concentrations in the highest quality anthracite and very high amounts in lignite, generating large volumes of SOx. As well as the need to treat the fuel prior to firing and control closely the combustion process itself to limit the production of nitrogen oxides, coal-fired stations based on conventional pulverized coal technology can only reduce SOx emissions through the use of post-combustion treatments. Further problems still remain through the safe disposal of fly ash which can contain high levels of toxic compounds including heavy metals.
d. Enormous environmental problems faced by operators of older, coal-fired generating plants all over the world, plants were forced to take drastic action after various public protests about the deadly effects of SOx emissions and other emissions. Emissions from coal and lignite-fired units at various power generating stations caused widespread damage, killing livestock and crops downwind of the plant and causing respiratory illness in the population in many countries. The plants were forced to cut output. This tends to place an unacceptably high strain on the commercial viability of an existing power station in many of the developing nations and represents a completely uneconomic option for the majority of obsolescent installations. Power producers in industrialized developed countries are therefore adopting a variety of leading-edge clean-coal technologies for electricity generation.
e. New clean coal technologies are providing an attractive and economically viable option to post-combustion systems. Applying the latest combustion, steam and process technologies in new power plant or upgrading existing coal-fired generating facilities provides significant improvements in thermal efficiency, reducing environmental impact and energy costs to the consumer. At the same time, higher thermal efficiencies result directly in reduced fuel costs, improving profitability and market position for the independent power producer.
(i) For new and smaller coal-fuelled generating plant, boilers using well-proven circulating fluidized-bed CFB technology provide a cost-effective and efficient system capable of meeting current and future environmental standards. They are now being widely used and successfully operated in coal-fired generating units, burning a very wide range of coal and other fuels with widely differing heat values and mineral content. These can typically include anthracite, semi-anthracite, bituminous and sub-bituminous coal, lignite and even ‘gob’ – a form of high-ash bituminous coal waste.
(ii) As an alternative to direct combustion based systems, coal gasification is becoming increasingly attractive, with Integrated Gasification Combined Cycle (IGCC) technology offering one of the best ‘clean’ options for effective power production. Gasification systems use steam and controlled amounts of air or oxygen under high temperatures and pressures to react with coal to form clean synthetic gas or ‘syngas’. Current systems provide efficient clean-up of the gas-stream to produce a mixture of carbon monoxide and hydrogen which can be used subsequently as a ‘clean’ fuel or a basic feedstock for liquefaction.
f. Used as a fuel for power generation in a typical IGCC generating plant, a syngas-fired gas turbine drives a generator, with exhaust heat from the gas turbine recovered to produce steam to power a steam turbine in conventional ‘combined cycle’. IGCC power generating systems are presently being developed and operated in Europe and the
g. As an alternative to the direct use of syngas as a fuel for electricity generation, it can also be processed using modern gas-to-liquids (GTL) technologies to produce a wide range of liquid hydrocarbon fuels such as gasoline and diesel oil. Coal-to-oil is a long-established technology in coal-rich
h. Nevertheless, clean coal technology is moving very rapidly in the direction of coal gasification, with a second stage designed to produce a concentrated and pressurized carbon dioxide stream followed by separation and geological storage. This has the potential to provide extremely low emissions of conventional coal pollutants, and as low-as-engineered carbon dioxide emissions – a vital step in the fight to prevent irreversible climate change.